Under-exposure Versus Higher ISO?

Discussion in 'Digital Photography' started by Mardon, May 5, 2006.

  1. Today Kaz Kylheku attempted to dazzle everyone with this
    profound linguistic utterance
    Definitely.
    This is not at all a criticism of the OP, but what you say is
    also true of over-exposed images, which is what Mardon's
    pictures look like to me.

    You should always sample signals using
     
    All Things Mopar, May 5, 2006
    #21
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  2. Mardon

    Scott W Guest

    Ok here are a bunch of shots from the 20D and the XT at a number of
    ISOs.
    These have been converted from raw with no noise filtering in place. A
    bit of
    filter could make them look somewhat better but detail would be loss.

    http://www.pbase.com/konascott/isotest

    As I said before I would be pretty happy with the XT at ISO 400 and the
    20D at ISO 800.

    But a lot depend on what I am going to do with the photos, if printed
    as 8 x 12 the higher ISO numbers would probably be ok.

    Scott
     
    Scott W, May 5, 2006
    #22
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  3. It is better to let the camera apply "analog gain" before
    quantization, than to boost the result in post-processing. Analog gain
    will produce less noise than postprocessing.
    Since you shoot Raw, never use ISO 3200. It does the same as
    postprocessing an ISO1600 on the 20D.

    Bart
     
    Bart van der Wolf, May 6, 2006
    #23
  4. The answer to the question has to do with the sensor gain and
    the sensor read noise. Both of these are summarized in tables
    1 and 2 at:
    http://www.clarkvision.com/imagedetail/digital.signal.to.noise

    For an XT (350D), the gain at iso 800 is 1.3 electrons/DN
    (DN = 12-bit digital number from the camera). The gain
    is 0.6 electron/DN at iso 1600. These is no effective point in
    digitizing beyond 1 electron. Then add read noise. The read
    noise of the 350D is 4.9 electrons at iso 800 and 3.7 at
    iso 1600. These is a modest improvement at iso 1600, but
    by the time you have enough electrons for your image,
    the image is photon noise limited (once you have a few tens
    of electrons). Thus for practical images there would barely
    be a perceptible improvement from 800 to 1600.
    If you were shooting faint sources (low light night images,
    the differences would be a little more apparent, but still
    small).

    The advantage of iso 800 is you have double the dynamic range
    as that at iso 1600, and almost the same noise.

    Roger
     
    Roger N. Clark (change username to rnclark), May 6, 2006
    #24
  5. Mardon

    Neil Ellwood Guest

    Why would we suppose this?
    Why don't you get a 20d and try it?
     
    Neil Ellwood, May 6, 2006
    #25
  6. Mardon

    Mardon Guest

    To be helpful.
    I already own one. That's why I asked the question. I want to know
    which method will produce the best final image.
     
    Mardon, May 6, 2006
    #26
  7. I think his point would be: Why don't you (also) do this and post the
    results? If others have or will do this, then the answers may add up to
    something useful.
     
    John McWilliams, May 6, 2006
    #27
  8. Mardon

    Mardon Guest

    I'm not good at doing 'rigorously' controlled experiments. I
    already gave my personal impressions in an earlier post to this
    thread as follows:
    "My impression based on my everyday experience is that if a RAW image
    is underexposed by more than a full f stop, then it's probably better
    to go to the higher ISO. If the underexposure is less than a full
    stop, I think it's better to shoot at the lower ISO and use Noise
    Ninja or some other noise filter to reduce the noise. (BTW, I find
    Noise Ninja better than the noise reduction filter built into PS
    CS2.) I'm not sure of this conclusion , however, and wanted to find
    out what others think"
     
    Mardon, May 6, 2006
    #28
  9. O.K. totally fair; I am not, either.
    JPS has both the ability to do the testing - I think he already has- as
    well as write about it. Haven't seen him here recently, but I look
    forward to what he writes- maybe this time I will grasp more of it....

    Based on a loose understanding of reports and tests, I tend to set to
    ISO 200 as default, and don't go over 800 ISO. In some sports shooting,
    I will set to M, at 160 or 200 shutter, and wide open. When it gets
    darker , I'll stick on the 580EX, set to 250 and close down a stop or
    two. I am certainly open to better ways to do it.
     
    John McWilliams, May 6, 2006
    #29
  10. Mardon

    Neil Ellwood Guest

    Then try it .
     
    Neil Ellwood, May 7, 2006
    #30
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